This invention relates in general to plastifying devices and in particular to a new and useful screw plastifying and conveying device of an injection molding machine for plastics.
The invention relates to a screw plastifying and conveying device of an injection molding machine for plastics in which the plastification and transport of the plastic compound takes place on account of the rotary motion of the screw, and the injection of the plastic compound into the mold cavity takes place on account of the axial motion of the screw or by means of a separate injection cylinder. According to the known state of the art, the plastifying screws are divided into 3 zones, namely the so called feed or pull-in zone which is charged with granulate, therefore have a large pitch volume, the compression zone, in which the pitch volume decreases steadily in order to achieve thereby a compaction of the developing plastic melt, and the measuring or metering zone which has a constant pitch volume and serves the purpose of uniform transport while at the same time taking care of the homogenization of the plastic melt. The pitch volume ratio of the metering zone and pull-in zone is called the compression ratio and must be adapted to the properties of the plastic to be processed. Now, in order to widen the applicability of one and the same screw, dosing devices are known which deliver an adjustable amount of plastic granulate to the pull-in zone of the plastifying screw during its rotation (see e.g. Der. Spritzgiessprozess (The Injection molding Process), VDI-Verlag, Dusseldorf 1979, pages 67 to 89, German OS No. 1801259; German Pat. No. 2029353). The dosing device permits a reduction of the compression ratio below the value given by the screw geometry in that less material is fed to the plastifying screw than it is capable of conveying. Due to this "underdosing", the pull-in zone and a part of the compression zone are filled only partly with material. The compression zone is effective only in the area in which the screw threads are filled completely, wherefrom results the desired variable compression ratio.
But the disadvantage of these known devices is that the supply of shearing heat to the metering zone cannot be influenced because its length is fixed by the screw geometry and because this zone must always be filled completely. The consequence thereof is that, considering the shallow metering zone thread depths required for perfect homogenization of the melt, the heat input through viscous friction becomes too great so that the plastifying cylinder must be cooled in the metering zone area (see e.g. German Pat. No. 2029353, section 8, lines 5 to 15). In addition, practical experiments have shown that the conveying action of the known three-zone plastifying screws becomes unstable from a certain underdosing on, resulting in greatly varying plastification times and melt temperatures. This restricts the applicability of known screws greatly, which means that only a weak underdosing is possible. Nor is it possible to reduce the melt volume in the metering zone by underdosing to adapt thereby the melt dwelling time to the respective material properties, which is necessary especially whenever the material must be plastified near the upper limit of the permissible processing temperature.
This is necessary, for example, when the orientations and internal tensions in the molded part must be minimal or when glass-clear materials must be processed into molded parts having defined, exactly reproducible optical properties.